There is a great interest in the nuclear energy industry for construction materials which will absorb, and therefore not release, neutrons, e.g. in containers for waste fuel. These containers are predominantly made of aluminum (Al)-based materials, and boron (B) is a commonly used element for neutron absorbing. Boron can be typically incorporated into Al as B4C, TiB2 or simply B that forms AlB2 or AlB12 in an Al-matrix.
The manufacture processes for making such storage containers includes methods of direct chill (DC) casting, extrusion and rolling. These methods quite often produce process scraps of Al—B4C composite materials that can make up to 65% of the original source material.
In the interest of economic savings and waste reduction, it is therefore of great interest to find a way to reclaim the valuable process scrap. However, B4C particles tend to be thermodynamically unstable in liquid aluminum, causing interfacial reactions between B4C and Al. This significantly reduces the fluidity and castability of the liquid composite during remelting, making the process of reclaiming scrap materials and their subsequent casting extremely difficult.
Boron-containing, aluminum materials are also highly reactive in the molten state, and the reactivity is strongly temperature and time dependent. Therefore, in attempts to recycle the scrap material by simply melting, the surface temperature of the scrap becomes elevated (e.g. more than about 770° C.), resulting in the B4C-containing refractory reacting with the aluminum matrix, rendering the material useless for subsequent casting, as fluidity becomes reduced.
Attempts to combine “virgin”, or unused, cast composite material and composite scrap materials have also been unsuccessful since the virgin cast composite has to be heated to a relatively high temperature to permit a reasonable quantity of scrap to be incorporated into the melt within a reasonable time. This heating tends to cause the virgin composite to deteriorate.
Alternately, the temperature of the virgin composite can be kept at a lower level, but in this case, the incorporation rate of scrap material must be reduced, resulting in an increase in the time needed to recycle a given quantity of scrap material, and again the material tends to deteriorate.
U.S. Pat. No. 3,955,970 (Claxton et al.) teaches a continuous melting process involving submerging scrap material into molten aluminum at ratios of from 1:10 to 1:50 scrap material to molten aluminum. Stirring is used to incorporate the solid scrap into the molten aluminum. There is no specific teaching of ways to prevent reactions between the B4C in the scrap material and the molten aluminum matrix. U.S. Pat. No. 4,571,258 also teaches a method of reclaiming scrap material by submergence in molten aluminum, in this case the scrap being light gauge aluminium. This patent is silent on dealing with scrap material that contains B4C and methods of preventing B4C degradation or reaction with molten aluminum.
U.S. Pat. No. 6,223,805 describes a method for recycling metal matrix composites and includes, as options, mixing of the composite with “virgin” composite or with matrix metal. The patent focuses on a method of eliminating inclusions in the recycled material.
It is therefore highly desirable to establish a method of reclaiming Al—B4C composite scrap material while maintaining integrity of the B4C and fluidity in the resultant composite product.